Tailor-made ceramics are becoming increasingly important in industry. In many technical areas, ceramics optimized to the specific requirements are key materials without which many technologies would not be technically feasible. Modern high-performance ceramics therefore also differ fundamentally in their properties from the generally known, classical ceramics. Like these, they consist of nonmetallic, inorganic materials. However, they are produced synthetically under clean and controlled conditions and they acquire their specific properties only as a result of this.
In general, technical ceramics can be divided into two large groups. Firstly, these are the structural ceramics which are designed in principle to retain their shape and structure even under strong mechanical, biological, chemical or thermal load. The other subgroup comprises the functional ceramics. They have special properties. These properties are, for example, optical, electrical, dielectric and magnetic functions.
The material class consisting of the functional ceramics, in particular those having dielectric properties, occupies a special position. It has made a decisive contribution to the development of our industrial society through the varied properties of its materials. Functional ceramics have become extremely important owing to the rapid proliferation of microelectronics in recent years.
Electronic circuits now no longer manage without the implementation of dielectric functional ceramics. The efficiency of electronic circuits depends to a high degree on the efficiency of the dielectric ceramics used.
In particular, high-frequency applications in communication and sensor systems and in wireless data transmission require high-quality dielectric materials. Applications may be, for example, control systems of radar antennas with electronically controllable beam sweep. These use so-called phase shifters which make the radiation direction of phased array antennas electronically controllable. However, tunable high-frequency filters, modulators, amplifiers and oscillators are also possible for applications in mobile radio (GSM, UMTS, Bluetooth, W-LAN, etc.). Moreover, varied applications in contactless sensor technology are possible (e.g. RFID (radio frequency identification) applications). In said commercial applications, the materials used must additionally be economical, i.e. competitive in price compared with semiconductor components.
The quality and price requirements with regard to dielectric high-frequency materials are high.
In addition, the flexibility and mobility requirements with regard to the communication sensor systems are increasing. This results in the requirement for economical and dynamically reconfigurable high-frequency or microwave modules.
The demand for such controllable microwave components will increase in the years ahead.
However, a precondition for this is the provision of economical, controllable components which in turn depends directly on the quality and availability of the controllable, dielectric materials required for this purpose.
Possible materials for this purpose are the dielectric functional ceramics. In particular, ferroelectric oxide ceramics are suitable for these applications. They show a nonlinear dependency of the permittivity (relative permittivity) on the electric field strength, which is referred to as controllability. This effect can be brought about in them virtually without power with very short response times and with simultaneous transmission of higher-frequency powers. In addition, ceramic layers offer the possibility of planar system integration in order to meet the space requirements of microelectronics. In contrast, phase shifters based on coils having ferrite cores and based on PIN diodes have not become established owing to insufficiently fast response times and excessively large dielectric losses and a lack of planar integratability. The good planar integratability is made possible by use of the planar shaping methods for thin and thick layers such as, for example, chemical deposition methods (chemical solution deposition (CSD), chemical vapor deposition (CVD)) and physical gas deposition methods (physical vapor deposition (PVD)) for the thin layer production and ceramic screen printing or ceramic film casting for the thick layer production.
Among the ferroelectric oxide ceramics, inter alia the mixed oxide systems barium strontium titanate (Ba1-xSrxTiO3, BST), barium strontium zirconate titanate (Ba1-xSrxZryTi1-yO3, BZT) and the silver tantalate niobate (AgTaxNb1-xO3, ATN) system have already been tested with regard to their fundamental suitability as controllable dielectrics. Their dielectric properties were investigated using solid ceramic bodies, thick ceramic layers and thin layers. Said work on the thick and thin layers is, however, limited substantially to questions relating to measurement.
The BST system proved to be the most promising one.
However, the commercially available controllable components are based on thin BST layers which are produced by means of gas-phase deposition and have the disadvantage that their production is very complicated. In addition, the difficulty in establishing a defined stoichiometry in the production of thin layers is disadvantageous. Furthermore, the dielectric properties of thin layers are subject to a strong influence of internal stresses and of lattice parameter differences relative to the substrate. Thus, high lattice stresses due to differences in the coefficients of thermal expansion as well as the lattice parameters of layer and substrate can lead to reduced permittivity and controllability. In order to make this controllable or to minimize it, expensive single-crystal substrates are often used as substrates for thin layers or additional buffer layers are applied between substrate and layer, which gives rise to additional processing costs.
In contrast, components which are based on thick ceramic layers as functional layer have the advantage that their properties are determined virtually solely by the properties of the ceramic powders used and not by the substrate. Moreover, they can be produced economically and in large quantities via the screen printing technology already established in electronics. Moreover, it is possible to use more economical, polycrystalline substrates since a polycrystalline layer without preferred crystallographic orientation or epitaxy is in any case applied thereby. In principle, thick layers can be integrated on LTCC substrates (low temperature cofired ceramics) by the screen printing technique, which thick layers are being increasingly used in mobile radio technology and automotive electronics.
In comparison with thin BST layers, however, thick BST layers have to date shown, at frequencies above 5 GHZ, very greatly increased dielectric losses which additionally increase much more strongly than those of the thin layers at higher frequencies.
A small particle size leads firstly to reduced permittivities and secondly to an increasingly diffuse phase transition of the ferroelectric-paraelectric phase transformation at the Curie point. This has the positive secondary effect that the thermal stability of the permittivity is increased thereby.
In addition to said factors influencing the dielectric properties, deviations from the stoichiometric composition and impurities can also influence these significantly. They can in certain circumstances lead to a strong shift in the Curie point, to flattening of the permittivity curve or to reduction of the dielectric losses.
Small amounts of foreign ions, so-called dopants, are therefore often added to the ceramic in order to influence the properties in a targeted manner. In the literature, descriptions of thick BST and BT layers have to date been limited to undoped thick layers. Accordingly, it is not known how dopants affect the HF losses in thick BST layers.
Endo et al. in Journal of Materials Science 25 (1990) 619-623 and Nekrasov et al. in Inorganic Materials, 1970, Vol. 6, pages 1907 to 1909, disclose the fluoridation of undoped barium titanate (BT). The fluoridation is not thermally stable, which means that the material cannot be sintered.
Acceptor-fluoride-codoped PZT for US converters is disclosed by Eyraud et al. in Ferroelectrics, 1996, Vol. 175, pages 241-250. Applications as controllable dielectric for microwave frequencies are not mentioned.
The fluoridation of BT via the gas phase is disclosed in U.S. Pat. No. 3,111,414 but the production of fluoridated BT with metallic codoping is not possible according to this patent. There is just as little indication of controllable behavior and virtually no data on dielectric properties.
Hoh et al. in Journal of The American Ceramic Society Vol. 46, No. 11, pages 516-518 disclose Cr—F-codoped BT and the production of doped BT from Cr2O3 and CrF2 but give no information as to which composition finally prevails in the sample (F can become volatile as HF gas during production/sintering under humid air).
The fluoridation of BT via the mixed oxide route is disclosed in the master's work of Florian S. Paul, with the title “Fluoridation of Barium Titanate (BaTiO3) Ceramics”, Manchester Materials Science Centre, University of Manchester and UMIST. However, merely low-frequency properties of Mn—F- and Co—F-codoped BT are discussed there without any indication of the high-frequency/microwave properties and controllable behavior being given.
The separation of BaF2 during the sintering of BT without acceptor doping is described by Fujihara et al. Applied Surface Science 221 (2004) 178-183. There too, there is no information about controllable properties.
The doping with fluorine is also disclosed by Makovec et al., Journal of The American Ceramic Society 86 [3] 495-500 (2003). However, large losses at 1 kHz are also described there and there is no information regarding dielectric properties, only resistance measurements being mentioned.
All HF investigations on thick layers have to date been unsatisfactory. Thick layers have to date shown extremely large losses in the HF range. Thin layers have to date been substantially better but firstly they are also substantially more expensive to produce than thick layers and secondly they too show relatively large dielectric losses in the HF range so that they cannot be used commercially at present.
Conclusions about the HF behavior from the low-frequency behavior are not possible or not possible in a satisfactory way since in particular the losses in the HF range are orders of magnitude higher than in the NF range.
Finally, U.S. Pat. No. 5,427,988 and U.S. Pat. No. 5,635,434 disclose BST for ferroelectric composites, but exclusively in combination with magnesium compounds.